This invention relates generally to free piston Stirling cycle engines, heat pumps and coolers and more particularly relates to a free piston alpha type of Stirling machine that, although it has two pistons, is configured so that it can be operated with a phase angle between the pistons that provides thermodynamically efficient operation without a mechanical drive linkage between the two pistons. The principal advantage of the invention is that it can be manufactured at a substantially reduced cost because it allows wider concentricity and alignment tolerances than physically comparable Stirling machines.
As well known in the art, in a Stirling machine a working gas is confined in a working space that includes an expansion space and a compression space. The working gas is alternately expanded and compressed in order to either do mechanical work or to pump heat from the expansion space to the compression space. The working gas is cyclically shuttled between the compression space and the expansion space as a result of the motion of one or more power pistons and, in some machines a displacer piston. Historically the pistons were mechanically linked together by a drive mechanism, such as a crank, that rigidly confines the reciprocating pistons to a fixed phase relationship and a fixed stroke. Later Stirling machines have pistons that are “free” because their phase and stroke is not fixed by a mechanical linkage but instead the pistons are linked by forces applied by internal gases and springs and therefore their stroke can vary under different operating conditions. The compression space and the expansion space are connected in fluid communication through a heat accepter, a regenerator and a heat rejecter, the heat acceptor and heat rejector being heat exchangers. The shuttling of the working gas cyclically changes the relative proportion of working gas in each space. Gas that is in the expansion space, and gas that is flowing into the expansion space through a heat exchanger (the accepter) between the regenerator and the expansion space, accepts heat from surrounding surfaces. Gas that is in the compression space, and gas that is flowing into the compression space through a heat exchanger (the rejecter) between the regenerator and the compression space, rejects heat to surrounding surfaces. The gas pressure is essentially the same in the entire work space at any instant of time because the expansion and compression spaces are interconnected through a path having a relatively low flow resistance. However, the pressure of the working gas in the work space as a whole varies cyclically and periodically. When more of the working gas is in the compression space, heat is rejected from the gas. When more of the working gas is in the expansion space, the gas accepts heat. This is true whether the machine is working as a heat pump or as an engine. The only requirement to differentiate between work produced or heat pumped, is the temperature at which the expansion process is carried out. If this expansion process temperature is higher than the temperature of the compression space, then the machine is inclined to produce mechanical work so it can function as an engine. If this expansion process temperature is lower than the compression space temperature and the Stirling machine is driven by a prime mover, then the machine will pump heat from a cold source to a warmer heat sink.
In a Stirling engine the expansion space is often referred to as the hot space and the compression space as the cold space because the expansion space is at a higher temperature than the compression space. In a Stirling machine that is mechanically driven to pump heat, the temperature relationship of those two spaces is the opposite. Similarly, a piston that has an end face as a boundary of the expansion space is often called the hot piston in an engine and is the colder piston in a Stirling machine operating in a heat pumping mode. The opposite terminology is used for the pistons when operating in the two possible different modes. To avoid the confusion caused by naming the pistons after their relative temperatures, more consistent terminology is to use the term “expansion piston” for a piston that bounds an expansion space and the term “compression piston” for a piston that bounds a compressions space.
A Stirling machine that pumps heat is sometimes referred to as a cooler when its purpose is to cool a mass and is sometimes referred to as a heat pump when its purpose is to heat a mass. The Stirling heat pump and the Stirling cooler are fundamentally the same machine to which different terminology is applied. Both transfer heat energy from one mass to another. Consequently, the terms cooler/heat pump, cooler and heat pump can be used equivalently when applied to fundamental machines. Because a Stirling machine can be either an engine (prime mover) or a cooler/heat pump, the term Stirling “machine” is used generically to include both Stirling engines and Stirling coolers/heat pumps. They are basically the same power transducers capable of transducing power in either direction between two types of power, mechanical and thermal.
Stirling machines have long been categorized into three distinct types of configurations. They are the alpha, the beta and the gamma.
An alpha Stirling machine has two separate power pistons, one is an expansion piston (hot piston in an engine) and the other is a compression piston (cold piston in an engine). In previously known alpha Stirling machines, these pistons and their associated expansion and compression spaces are located in two different and separated cylinders. FIG. 1 is a diagrammatic illustration of the earlier alpha configuration in which two single acting pistons A reciprocate in cylinders B which contain their respective compression and expansion spaces C and D. The machine is single acting because only one end face of each piston interfaces the working gas. The compression and expansion spaces C and D are connected to each other through a heat accepting heat exchanger G that transfers heat from an external source into the working gas passing through it, a regenerator E, and a heat rejecting heat exchanger F that transfers heat from working gas passing through it to an external sink. A characteristic of the single acting alpha configuration is that the reciprocation of each of its pistons varies the volume of only its associated space. The expansion piston varies only the volume of the expansion space and the compression piston varies only the volume of the compression space. In the historically earlier alpha machines, the piston rods of the two pistons were linked together by a mechanical drive mechanism, such as a crank mechanism, that constrained the reciprocation of the pistons to a desired relative phase so that the working gas in and between the compression and expansion spaces would go through a thermodynamic cycle that permitted the machine to operate and do so efficiently. However, these mechanical drive mechanisms also permitted the pistons to reciprocate at a single, fixed stroke length.
FIG. 2 illustrates two pistons and cylinders of a free piston, alpha Stirling machine configuration that is well known in the art. Such Stirling machines having three or more pistons and cylinders have been described in the prior art. The pistons are double acting because one end face of each piston varies the volume of the expansion space in its cylinder and the opposite end face varies the volume of the compression space at the opposite end of its cylinder. As known in the art, the relative phase of the pistons must be 360° divided by the number of pistons. Consequently, if an alpha machine is constructed from the components illustrated in FIG. 2 and has only two pistons, the pistons would be phased 180° apart. However, 180° degree phasing of the pistons is thermodynamically inoperable. Insofar as known, the current technology does not describe a two piston, free piston alpha Stirling machine.
The second recognized Stirling machine configuration is the beta Stirling machine. The beta configuration has a single power piston arranged coaxially with a displacer piston. The displacer piston does not extract any power from or contribute any power to the working gas but only serves to shuttle the working gas between the expansion and compression space through the heat exchangers and regenerator.
The third recognized Stirling machine configuration is the gamma Stirling machine. It is much like a beta Stirling machine except the power piston is not mounted coaxially to its displacer piston. This configuration produces a lower compression ratio but is often mechanically simpler and often used in multi-cylinder Stirling engines.
Each of these Stirling machine configurations has its own set of advantages and disadvantages relative to the others. The alpha configuration is an assembly of relatively simple pistons in cylinders and requires relatively simple, and therefore relatively inexpensive, machining of its cylinders and pistons. However, the alpha configuration has multiple cylinders and it is impractical to house the multiple cylinders of an alpha machine in a single casing. Because it has multiple cylinders, the alpha configuration is not as compact as the beta configuration and also requires a surrounding assembly of a regenerator and associated heat exchangers for each of its multiple cylinders. Additionally, so far as known, a free piston, two piston alpha Stirling machine has not previously been possible.
The beta configuration is more compact and can be housed in a single casing because it has a single cylinder or two end to end adjacent, axially aligned, cylinders. The beta configuration has only one surrounding assembly of a regenerator and heat exchangers. A load (such as an alternator) or a prime mover (such as an electromagnetic linear motor) is easily connected to the single power piston of a beta configuration and can be housed in the same casing. Unfortunately, the beta configuration also has critical, small tolerance alignment and concentricity requirements in order to avoid problems of sealing the displacer's connecting rod to the piston, and rubbing of the displacer piston on the walls of its cylinder. Meeting these requirements and avoiding the problems requires higher precision machining which translates into higher costs of manufacture. The beta configuration ordinarily requires a relatively stiff spring for efficient operation but such a spring adds cost and a significant risk of failure from fatigue.
Although the advantages of the invention can be appreciated only after the invention is explained, it is an object and purpose of the invention to provide in one Stirling machine many of the desirable characteristics of the different Stirling machine configurations without many of the undesirable characteristics.
The principal object and feature of the invention is to provide a Stirling machine configuration that has performance characteristics comparable to existing Stirling machines but has a reduced manufacturing cost because it has reduced concentricity and alignment requirements.
Another object and feature of the invention is to provide a two piston, free piston, Stirling machine in an alpha configuration that is capable of being operated with the pistons reciprocating at a thermodynamically desirable phase relationship.
Yet another object and feature of the invention is to provide such a two piston, free piston alpha Stirling machine in which the phase relationship can be selectively tuned by the designer over a broad range of phase angles.